Recently UWB receives attention as new communication means. In UWB, large-capacity data transfer can be realized within a short range of about 10 m by the use of a wide frequency band. For example, a guideline of US FCC (Federal Communication Commission) has a plan to use a frequency band of 3.1 GHz to 10.6 GHz. One of the features of UWB is that the remarkably wide frequency band is used.
Recently a study on an ultra-wide-band filter that can be used for UWB is actively made. For example, there has been reported that a wide-band characteristic of a passband width exceeding 100% in terms of fractional band width (band width/center frequency) is obtained with a bandpass filter in which a principle of a directional coupler is applied (for example, see Non-patent Document; “Ultra-Wide-Band Bandpass Filter with Micro Strip-cpw Broadside Coupling Structure”, IEICE Proceedings (March, 2005) c-2-114, P. 147).
On the other hand, a bandpass filter in which a plurality of quarter-wave stripline resonators are provided in parallel while mutually coupled is well known as a filter frequently used conventionally (for example, see Japanese Patent Publication Laid-Open No. 2004-180032).
However, because the bandpass filters proposed by Non-patent Document and Japanese Patent Publication Laid-Open No. 2004-180032 have problems, and the bandpass filters are not suitable to the UWB bandpass filter.
For example, unfortunately the passband width is excessively wide in the bandpass filter proposed by Non-patent Document. That is, basically a frequency band of 3.1 GHz to 10.6 GHz is used in UWB, and International Telecommunication Union Radiocommunications Sector establishes a plan that the frequency band is divided into a Low Band and a High Band while a frequency of 5.3 GHz used in IEEE 802.11.a is avoided. The frequency band of about 3.1 GHz to about 4.7 GHz is used in the Low Band, and the frequency band of about 6 GHz to about 10.6 GHz is used in the High Band. In Japan, there has been a plan to initially use the Low Band. Therefore, because a passband width of about 40% in terms of fractional band width and attenuation at the frequency of 5.3 GHz are simultaneously required for the filter used in the Low Band, the bandpass filter proposed by Non-patent Document with the passband width exceeding 100% in terms of fractional band width cannot be used due to the excessively wide passband width.
The conventional bandpass filter in which the quarter-wave resonator is used has an excessively narrow passband width, and the fractional band width is lower than 10% even in the bandpass filter disclosed in Japanese Patent Publication Laid-Open No. 2004-180032 that is aimed at the wide band. Therefore, the bandpass filter disclosed in Japanese Patent Publication Laid-Open No. 2004-180032 cannot be used for the UWB bandpass filter in which the wide passband width corresponding to the fractional band width of 40% is required.
Further, in the bandpass filter for the Low Band of UWB, because of the demand for the attenuation at the frequency of 5.3 GHz used in IEEE 802.11.a and at the frequency band of 6 GHz to 10.6 GHz used in the High Band of UWB, it is particularly necessary to secure the adequate attenuation on a high frequency side of the passband.